Effect of the Pt–Pd molar ratio in bimetallic catalysts supported on sulfated zirconia on the gas-phase hydrodechlorination of chloromethanes

Abstract

Bimetallic Pt:Pd catalysts with different molar ratios and 0.5 wt% overall metal load supported on sulfated zirconia catalysts (SZ) were synthesized and tested in the gas-phase hydrodechlorination (HDC) of chloromethanes and their mixtures. The catalysts were characterized by adsorption-desorption of N2 at-196 ºC, X-ray diffraction, X-ray photoelectronic spectroscopy, temperature programmed reduction and aberration corrected scanning transmission electron microscopy (STEM). The effect of the Pt:Pd molar ratio on the activity, stability and selectivity was analyzed. The high acidity of the sulfated zirconia results in metal particles of small size (mainly < 5 nm) as confirmed by STEM. The bimetallic catalysts showed higher stability than the monometallic ones as demonstrated in long term experiments (80 h on stream), confirming the positive effect 2 of combining both metallic phases. TOF values in the range of 0.0007 to 0.0168 s-1 and apparent activation energies between ≈ 41-44 kJ•mol-1 were obtained. TOF values for dichloromethane HDC increased at increasing mean metal particle size within the range of this work (≈1.2-2.3 nm). The catalysts with Pt:Pd molar ratios of 1:3 and 1:1 showed significantly better performance than the 3:1 one for overall dechlorination due to their higher atomic metal content and TOF at the same total metal weight load (0.5%).

The Physico-Chemical Characterization of Mesoporous Sulfated Zirconia Nanoparticles Prepared under Structure-Directing-Free for &lt;i&gt;n&lt;/i&gt;-Hexane Isomerization

“…The small particle size is advantageous due to shorter diffusion distances for both reaction substrates and products, which increases reaction rates. The high acidity of the sulfated zirconia support could explain the small metal particle size, since the presence of acidic groups increases the hydrophilic character of the support and favors the diffusion of the metal precursor [6]. A typical SAED pattern is shown in Figure 5 because a good match is observed with tetragonal structure, i.e., the obtained tetragonal lattice value was 0.294 nm in agreement with XRD analysis.…”

“…The increased sulfate groups not only favour stabilizing the active tetragonal phase of zirconia but also increasing the total surface area and pore volume, and hence enhancing the Brønsted and Lewis acidities, and increasing the oxidative dehydrogenation. The high acidity of the sulfated zirconia support could explain the small metal particle size (as will be corroborated later by the FETEM observations), since the presence of acidic groups increases the hydrophilic character of the support and favors the diffusion of the metal precursor [6].…”

“…The accurate P d superficial concentration is not available due to its low loading and partial overlap of P d 3d 5/2 with Zr 3p 3/2 [14]. Only weak signal exists for the 0.5 wt% P d catalyst because of the low concentration of P d atoms [6] [15] [16].…”

“…Therefore, based on the clue given by the EDXS analysis, we have quantitatively determined the metal and non-metals by ICP-AES and by elemental analysis and X-ray fluorescence spectroscopy, respectively, yielding (wt%) Zr (51.85), S (2.388), H (3.15), N (0), and Cl (0). Combined with the data derived from the thermal gravimetric and elemental analyses, the pristine complex can be represented by an empirical formula: Zr 4 (S 2 O 7 ) 0.25 O 2 (OH) 5 (CH 3 COO) 6 •H 2 O. Figure 10 shows the ion current curve for evolved gases in the TG-MS measurement of the catalyst.…”

Influence of microwave activation on the catalytic behavior of Pd-Au/C catalysts employed in the hydrodechlorination of tetrachloromethane

“…An efficient way to modify the electronic structure is to add dopant [6][7][8]. Alloy formation usually affects selectivity, activity and stability of catalysts used in HDC.…”